The present invention relates to a thin-film magnetic field sensor for measuring a magnetic field in a space, and particularly to a thin-film magnetic field sensor for precisely measuring the degree and direction of a magnetic field, using a giant-magneto-resistant thin-film, such as a nano-granular giant-magneto-resistance effect thin-film.
FIG. 1 shows a magnetic field sensor disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 11-87804 and 11-274599. In FIG. 1, the portion indicated with xe2x80x9cGiant-magneto-resistant thin-filmxe2x80x9d denotes a nano-granular giant-magneto-resistant thin-film of xe2x80x9cMetal-Insulatorxe2x80x9d, which renders an electrical resistance change of about 10% when a magnetic field of 10 k Oe is applied. As in this example, giant-magneto-resistant thin-films have a larger range of change in the electrical resistance value, as compared to ordinary magneto-resistance effect materials. However, as described above, they require a large applied magnetic field to cause an electrical resistance change. Accordingly, where a giant-magneto-resistant thin-film is solely used, hardly any change in the electrical resistance value can be expected with a small magnetic field of about 100 Oe or less, which is generally used in magnetic field sensors.
The structure shown in FIG. 1 is arranged to compensate for the problem described above. Specifically, there are soft magnetic thin-films, which function as members for gathering magnetic flux around them. Where the dimensions of-the soft magnetic thin-films are appropriately selected, the giant-magneto-resistant thin-film can be supplied with any degree of large magnetic flux density within the saturation magnetic flux density of the soft magnetic thin-films, regardless of the degree of the magnetic field around the soft magnetic thin-films, in principle. Furthermore, seeing the structure shown in FIG. 1 from the point of view of electrical resistance, although the electrical resistance value between the soft magnetic thin-films is the sum of those of soft magnetic thin-film portions and a giant-magneto-resistant thin-film portion, the resistance value between the soft magnetic thin-films is substantially equal to the resistance value of the giant-magneto-resistant thin-film portion, because the giant-magneto-resistant thin-film has a high electrical resistivity, which is 100 times or more that of the soft magnetic thin-films. In other words, the electrical resistance value of the giant-magneto-resistant thin-film directly takes on the electrical resistance value between the soft magnetic thin-films. FIG. 2 shows an example of change in electrical resistance in the structure shown in FIG. 1. As shown in FIG. 2, a change of about 6% in the electrical resistance value is realized with a small magnetic field of several Oe. This change is twice or more larger than those of anisotropic magneto-resistance effect materials, which are conventionally used.
However, it has been found that the structure shown in FIG. 1 brings about a big problem, where it is used to realize a magnetic field sensor for measuring the absolute value of an applied magnetic field on the basis of a measurement value of the electrical resistance of the giant-magneto-resistant thin-film. The problem relates to change in the magneto-resistance value of the giant-magneto-resistant thin-film due to temperature. As described above, in the case of the structure shown in FIG. 1, there is freedom of choice in the degree of a magnetic field to be detected. However, even if the sensitivity is increased, it is a choice relative to a magnetic field to respond, and it is impossible to obtain a range of change larger than change in the electrical resistance of the giant-magneto-resistant thin-film, in principal. Actually, in the case of the structure shown in FIG. 1, the change value in electrical resistance has been reduced due to other factors as well as that describe above, and becomes about 6%. If a change of the giant-magneto-resistant thin-film due to temperature is added to the change value of about 6% in electrical resistance, the additional change in electrical resistance for that can be a variable in estimating an applied magnetic field. FIG. 3 shows examples of a temperature characteristic. As shown in FIG. 3, change in the resistance value of the giant-magneto-resistant thin-film due to temperature is larger than change in the resistance value due to the applied magnetic field. Accordingly, the structure shown in FIG. 1, as it is, can be hardly used as a magnetic field sensor for measuring the absolute value of a magnetic field.
Furthermore, it has also been found that the conventional structure shown in FIG. 1 brings about a big problem, where it is used to realize a magnetic field sensor for measuring the absolute value and direction of an applied magnetic field. That is, change in the electrical resistance of the giant-magneto-resistant thin-film does not depend on the direction of a magnetic field, but has an isotropic characteristic. Specifically, as shown in FIG. 2, the structure shown in FIG. 1 provides the same change in electrical resistance in positive and negative directions of a magnetic field, and thus the direction of the magnetic field is hardly specified. Accordingly, the structure shown in FIG. 1, as it is, can be used as a sensor for only detecting the degree of a magnetic field, but cannot be used as a sensor for specifying the direction of the magnetic field, such as an azimuth sensor for reading the direction of geomagnetism, or a sensor for reading an angle relative to a polarized magnetic material.
An object of the present invention is to provide a thin-film magnetic field sensor, which has a simple structure and a high detecting sensitivity, and reduces measurement errors due to temperature variation or the like, and can measure the intensity and direction of a magnetic field.
At first, the present invention provides a thin-film magnetic field sensor comprising: a first arm including soft magnetic thin-films, which are separated by a gap having a predetermined gap length, and each have a predetermined film thickness and a predetermined width at a portion in contact with the gap, a giant-magneto-resistant thin-film formed to fill the gap between the soft magnetic thin-films, and terminals respectively and electrically connected to the two separated soft magnetic thin-films; and a second arm including conductive films, which are separated by a gap having a gap length substantially equal to said gap length, and each have a film thickness substantially equal to said film thickness and a width substantially equal to said width at a portion in contact with the gap, a giant-magneto-resistant thin-film formed to fill the gap between the conductive films, and terminals respectively and electrically connected to the two separated conductive films, wherein these arms form two arms of a bridge circuit.
Specifically, according to the present invention, in the electrical resistance value changes of a giant-magneto-resistant thin-film, changes due to temperature, humidity and time-varying are removed, and only a change due to a magnetic field is extracted, so that a magnetic field sensor with a high accuracy is realized. More specifically, a bridge is formed of two lines of elements, which have the same giant-magneto-resistant thin-film and structure, wherein one of the elements includes soft magnetic thin-films one on either side of the giant-magneto-resistant thin-film to increase the sensitivity relative to the magnetic field, and the other element uses the giant-magneto-resistant thin-film as it is to make the sensitivity substantially zero relative to the magnetic field. Since the output of the bridge is in proportion to the difference in the electrical resistance value between the elements, changes of the giant-magneto-resistant thin-film due to temperature, as well as other changes due to humidity, time-varying, etc. are removed from the output voltage, whereby only an electrical resistance value change due to a magnetic field appears in the output. As a result, it is possible to accurately detect the absolute value of the magnetic field, and also to detect a very small magnetic field.
Where two first arms each having soft magnetic thin-films one on either side of a giant-magneto-resistant thin-film, and two second arms each having conductive films one on either side of a giant-magneto-resistant thin-film are disposed to form a bridge circuit, the bridge output voltage can be twice that of the structure according to the first invention, whereby it is possible to realize a thin-film magnetic field sensor with a higher accuracy and a higher sensitivity to the magnetic field.
The present invention also improves the accuracy of a thin-film magnetic field sensor, in terms of used materials. Specifically, where giant-magneto-resistant thin-films used in elements forming a bridge have the same material and structure, but materials sandwiching the giant-magneto-resistant thin-films are different, a small difference may be brought about in the electrical resistance value, depending on contact potential difference, thermally electromotive force, etc. With an arrangement according to a third invention, electrical resistance changes of the giant-magneto-resistant thin-films due to causes including problems describe above other than a magnetic field applied to the two structures are strictly canceled, whereby it is possible to realize a thin-film magnetic field sensor with a still higher accuracy.
The present invention also realizes a thin-film magnetic field sensor with a more compact size and a higher accuracy, in terms of the structure. Specifically, in order to increase the sensitivity of a magnetic field sensor and to also make its shape compact, it is necessary for a structure, which has soft magnetic thin-films one on either side of a giant-magneto-resistant thin-film, to have the soft magnetic thin-films compact while maintaining a constant effective area of the soft magnetic thin-films. With an arrangement according to a fourth invention, it is possible to realize a thin-film magnetic field sensor with a high sensitivity and a compact shape.
The present invention also improves the accuracy of a thin-film magnetic field sensor, in terms of residual magnetization. Specifically, where there is residual magnetization in soft magnetic thin-films after an applied magnetic field is measured and the external magnetic field is removed, the residual magnetization has an effect on a giant-magneto-resistant thin-film similarly to an externally applied magnetic field, which lowers accuracy in detecting the magnetic field. Accordingly, where the soft magnetic thin-films are magnetized in a direction perpendicular to a detection magnetic field of the giant-magneto-resistant thin-film, residual magnetization in the soft magnetic thin-films is reduced, so that a magnetic field can be measured more accurately.
At second, the present invention provides a thin-film magnetic field sensor comprising: a magnetic field sensor element including soft magnetic thin-films, which are separated by a gap having a predetermined gap length, and each have a predetermined film thickness and a predetermined width at a portion in contact with the gap, and a giant-magneto-resistant thin-film formed to fill the gap; and a magnetic field generator configured to apply a bias magnetic field to the magnetic field sensor element.
Specifically, the magnetic field zero point in an electrical resistance change curve is arbitrarily shifted by applying the bias magnetic field, in order to create a difference in the electrical resistance change between positive and negative directions because of a difference in the magnetic field to be measured between positive and negative directions (polarities). With this arrangement, since there is a difference in the sensor output depending on the direction of a magnetic field, it is possible to determine the direction of the magnetic field. Furthermore, where a bias magnetic field corresponding to a magnetic field with the largest change in the electrical resistance change curve is applied, the sensitivity of the sensor is improved, as compared to the case where no bias magnetic field is applied.
Where a coil is used, the degree of the bias magnetic field is easily controlled by adjusting a current flowing in the coil. On the other hand, where a device has a small sensor, which hardly allows a coil to be formed, a soft magnetic thin-film and a hard magnetic film having a predetermined degree of coercive force may be stacked one on the other, so that a bias magnetic field is applied to a magnetic field sensor element by the hard magnetic film.
In a thin-film magnetic field sensor according to the present invention, internal distortions and stresses are left when films are formed. Accordingly, problems arise such that the sensor does not show the expected performance and noises become large. These problems in properties are solved, where a heat treatment is performed at a temperature of 50xc2x0 C. or more and 500xc2x0 C. or less after the film formation, so that the internal distortions and stresses are removed. Where the temperature is lower than 50xc2x0 C., the internal distortions and stresses are not sufficiently removed, while, where it is higher than 500xc2x0 C., properties of soft magnetic thin-films or a giant-magneto-resistant thin-film are deteriorated, and thus such temperatures are not appropriate.
Particularly, a magnetic field generator may be a coil would around soft magnetic thin-films and a giant-magneto-resistant thin-film. A magnetic field formed by a current flowing in this air-cored coil only follows Biot-Savart law, and thus a constant magnetic field can be always applied, in spite of temperature or time-varying, so long as the coil has a stable shape. With reference to the magnetic field with an accurate value, the intensity of a magnetic field therearound can be determined. In this case, the coil may be formed of a linear conductor or a thin-film conductor. With the coil, the direction of a magnetic field to be applied to the soft magnetic thin-films and giant-magneto-resistant thin-film can be selected by changing the direction of the flowing current between positive and negative directions. The direction of a magnetic field therearound can be determined with reference to this.
As regards means for detecting the resistance value between electrical terminals in a thin-film magnetic field sensor according to the present invention, which includes a coil would around soft magnetic thin-films and a giant-magneto-resistant thin-film, the resistance value may not be directly measured, but the measurement of the resistance value may be replaced with measurement of voltage, which is easier, using an arrangement where electrical terminals are disposed on one arm of a bridge circuit to measure a bridge output voltage.
A coil wound around soft magnetic thin-films and a giant-magneto-resistant thin-film can be realized, using a technique for a conductive thin-film wound around the soft magnetic thin-films and giant-magneto-resistant thin-film. By applying the technique for a conductive thin-film, a coil is realized to be close to the soft magnetic thin-films and giant-magneto-resistant thin-film. Since the intensity of a magnetic field generated when a current is caused to flow in the coil is in reverse proportion to the distance relative to the coil, the necessary current value to apply a predetermined magnetic field intensity to the soft magnetic thin-films and giant-magneto-resistant thin-film can be reduced, as the coil is closer. Since a current flowing in the coil is a dominant factor in the power consumption of the sensor, it is possible by this coil technique to realize a compact magnetic field sensor with a low power consumption.
Current values flowing in a coil would around soft magnetic thin-films and a giant-magneto-resistant thin-film film are selected to comprise two currents in positive and negative directions, which have substantially the same absolute value within a range where magnetization of the soft magnetic thin-films and giant-magneto-resistant thin-film does not reach saturation. The intensity and direction of a magnetic field are determined based on a difference in the resistance value between the cases of these currents being caused to flow. With this arrangement, fluctuations in the absolute value of the resistance value are removed by difference in the resistance value. Since the sign of the difference in the resistance value agrees with the sign of an externally applied magnetic field, the direction of the magnetic field can be easily determined.
Where a current flowing in a coil wound around soft magnetic thin-films and a giant-magneto-resistant thin-film has a value to substantially saturate the soft magnetic thin-films and giant-magneto-resistant thin-film, the value of a residual magnetic field becomes a forcedly decided magnetization value, and thus measurement errors due to residual magnetization can be solved.
The present invention also shows a specific arrangement to determine the accurate value and direction of an external magnetic field and to solve errors due to residual magnetization at the same time. Specifically, at first, a positive direction current, which substantially saturates the soft magnetic thin-films and giant-magneto-resistant thin-film, is caused to flow in the coil. With this operation, existing magnetization is solved and magnetization in one direction is forcedly provided in the soft magnetic thin-films and giant-magneto-resistant thin-film. Then, the current is continuously reduced from the value for saturation to a predetermined positive current value within a range which does not provide saturation, and, in this sate, a resistance value Rpp is measured between terminals. Then, the current value is continuously changed to a predetermined negative current value, and, in this sate, a resistance value Rpm is measured between the terminals. Then, a negative direction current, which substantially saturates the soft magnetic thin-films and giant-magneto-resistant thin-film, is caused to flow in the coil. With this operation, the magnetization described above is solved and magnetization in the opposite direction is forcedly provided in the soft magnetic thin-films and giant-magneto-resistant thin-film. Then, a predetermined negative current value within a range which does not provide saturation is applied, and, in this sate, a resistance value Rmm is measured between the terminals. Then, the current value is continuously changed to a predetermined positive current value, and, in this state, a resistance value Rmp is measured between the terminals. The absolute value and polarity of the intensity of a magnetic field around the magnetic field sensor are determined by ((Rpm+Rmm)/2xe2x88x92(Rpp+Rmp)/2) derived from the resistance values thus obtained, so that the accurate value and direction of an external magnetic field can be determined, while removing the influence of magnetization.